While I admit I'm not sure the exact ATF rules on the subject, it's could be considered a way to avoid having the background check done on yourself. This is the precise reason we don't allow it at BGF. Someone not related to the person filling out the 4473 cannot pay for the gun.
I dont blame you. On the other side of the counter, when my wife wanted to buy her first gun, we went in together and she picked it out. I helped, but when the rubber met the road, I stepped back and told her that it was her purchase so she was dealing with the gentleman behind the counter. She did it all and I just stood there and looked sexy.